


Human anatomy

by Moonie711



Category: Undertale (Video Game)
Genre: Aggression, Biology, Body Worship, Dom Sans, Dom/sub, Ecto-Penis (Undertale), F/M, Hair-pulling, Kissing, Lemon, Love Bites, Magic, Possession, Reader-Insert, Rough Sex, Smut
Language: English
Status: In-Progress
Published: 2017-02-11
Updated: 2017-07-08
Packaged: 2018-09-23 14:28:38
Rating: Not Rated
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 3
Words: 4,033
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/9661328
Author URL: https://archiveofourown.org/users/Moonie711/pseuds/Moonie711
Summary: Things take an unexpected turn when sans asks you to teach him more about the human body.Reader x SansNow updated with long descriptive lemon, jam packed with 20% more sin.Lets be honestThis whole story is nothin but smut and sin. ;D





	1. Hands on experience

"Sorry im late!" You apologized, bursting into the room unannounced, unsurprisingly Sans didn't seem to mind, he had been expecting you anyways. 

You couldn't help but smile as you caught the skeletons gaze, giving a small wink in greeting as you slung your backpack over your shoulder and onto the bed. "Alright, ready to get schooled?" you collapse onto the bed next to him, making him bounce slightly.

"well well, whats this?" The skeleton raised his brow bone as he rummaged through the contents of the bag, smirking slightly as he withdrew a science book. 

"To teach you! You said you were curious." You beamed at him, grabbing a book from the bag and flipping it to a random page. "See, this ones about human anatomy, I thought we could read it together tonight." 

"pfft-" Sans couldn't hold in the snort of laughter that escaped him, immediately replacing the grin on your face with a glare instead. He knew what he had said earlier, but didn't expect you to take him so literal.

"What the hell is so funny? Im being serious." 

"no, nothing. it's just, when i said i wanted to learn more about the human body-- well tibia honest this isn't exactly what i had in mind." He admitted, pointing to the books. your cheeks flushed a deep red at the realization of what he was saying.

"O-oh, o-of course you didn't.." You couldn't help but physically face-palm at the misunderstanding. 

How the hell didn't you realize it sooner?

Sans was flirting with you...

Despite feeling incredibly stupid and naive, you risk a look at the skeleton by peeking at him through your fingers, only to find the white lights of his eyes flickering back tauntingly in amusement.

"we can still read 'em, if you want." 

"Do you want to?" 

"yea. 'though-" He reached out, his hand ghosting past your cheek to gently grab at a lock of hair. "i think i could learn better from a more hands-on experience." he fidgeted with the lock of hair for a moment before redirecting his attention back to you, watching closely as your face twisted in embarrassment.

"W-wait, w-what?" You could feel your face flare up instantly, unable to form a single sentence as you kept stumbling over your words.

"humans make the funniest facial expressions." he observed lightly, unable to control his curiosity as his phalanges slowly wondered to your cheek and down your neck, only pausing as he got to your pulse point.

*jeeze her heart is beating like crazy..

"how come i can feel your pulse here, if your heart is actually located- here." He removed his hand from its place on your neck and pressed it up against your chest, feeling for your heartbeat.

"I-it's because of a main artery in my neck. It's responsible for carrying blood to my brain." You tried to ignore the position of his hand and how close it was to your breasts.

"and to your face." Sans snickered teasingly.

"Y-you can feel it here to." You tried your best to ignore him as you took his hand and guided it to your wrist, helping him find the pulse point there as well.

As soon as he felt it the lights of his eyes lit up with surprise, sitting in silence a moment as he felt the steady rhythm of your pulse. "what about your other arm?" He took the limb curiously, lightly pressing his phalanges over the same spot.

"I think I have a pulse point somewhere on my legs to." You thought out loud, instantly sans eyes flickered to your thighs, a devious grin plastered on his face. "D-don't get any ideas!" You felt your cheeks heat up again.

"come on, it's for science." He winked at you, successfully receiving a playful shove in response. 

"Thats what the books are for!" You protested half-heartedly, unable to hold back a giggle.

His smile softened as his hand wondered from the pulse on your wrist to your hand, entangling his phalanges with yours.

*her hand looks so small in comparison..

"does it feel weird to hold my hand?" His grip around yours tightened slightly, so you gave him a small squeeze back.

"I like it when you hold my hand, i don't think it feels weird, just different." You admitted honestly.

"heh, me to." Sans lazily dragged his fingertips along your arm as he traced it upwards, back towards your face, to finally cup your chin in his palm. You failed to control the butterflies that fluttered into your stomach as he leaned in close to you, his face mere inches from yours.

"knock, knock."

Really? A joke now, Of all times? 

You cant deny your curiosity, and against your better judgment, decide to humor him anyways.

"W-who's there?" 

"dewey."

"Dewey who?" You raised your eyebrows curiously, making the skeletons grin widen even more as he leaned in close to your ear.

"dewey have to use a condom?" 

His voice was deep, seductive, and serious- which made you laugh even harder as you busted into a fit of giggles. 

You quickly cupped a hand over your mouth as to not offend sans, but he didn't seem bothered, only amused. You weren't quite sure how to continue from there so you decided to reply with a joke of your own.

"Hey sans."

"ye?"

"Knock, knock." You smiled deviously at him.

"who's there?" He leaned in curiously, his grin widening slightly.

"Butch, Jimmy, and Joe."

His smile dropped, turning into a frown instead; clearly unsure where you were going with this.

"ok, Butch, Jimmy, and Joe, who?" 

You scooted into his lap.

"Butch your arms around me." You grabbed his hands and guided them to your waist.

"Jimmy a kiss." You leaned in and gave him a small peck on the mouth.

"And let's Joe." You gave him a playful wink as you leaned into him.

Now it was sans turn to blush as a small tint of blue covered his face, clearly not expecting you to play along with him.

"well damn girl, all you had to do was ask." He quickly recovered, flipping you over onto the bed, successfully pinning you beneath him. "don't think i'll be going easy on you just cause your a human." He warned, his voice suddenly darkening, sending a shiver of excitement down your spine.

"What better way to learn than to experience it first-hand rite?"

"exactly."


	2. Masters degree in Biology

**Summary for the Chapter:**

> Sans x Reader
> 
> WARNING: 
> 
> Long and descriptive lemon, now available with 20% more sin!

As sans trailed his bony fingers over your body you couldn't ignore the surge of power you felt radiating off of him, which was surprising. You had known Sans a long time and had never felt so much as a spark from him before this. You would see him teleport every once in awhile, but never became fully aware of his true power until now.

"Your so strong." You blurted out-loud, more to yourself than towards him. Sans default grin widened, looking almost sinister as he looked down at you.

"What makes you say that?" He continued to toy with you as he grid his pelvic bone into you, making you grit your teeth to prevent a moan from escaping.

"Y-you-- ah!" You let your head fall backwards as his motions were slow and teasing, making you melt underneath him. "You've never showed me before.." You finally managed as he paused his actions. "But I can feel it." You looked up at him, wondering if you were making enough sense for him to understand.

"Ye?" He smirked, groping your breast roughly before moving his hand back up to pin your arms above your head, a tinge of magic enveloped your already sensitive body as his actions became more aggressive. He leaned down, giving your earlobe an affectionate lick. "Are you scared?" His voice was dark, almost menacing, god why did it turn you on so much?

"N-no." You only half-lied, a part of you was curious and excited, the other...absolutely terrified to see his full potential.

Had he really been holding back this whole time?

"careful what you wish for sweetheart." He cooed in your ear, making you shiver in response to his nickname for you. You wish you could call him 'sanswich' and make it sound half as sexy as he had, but decided to stay silent and let him take control instead.

"what if this IS my wish?" You replied, actually wishing you could cover your face, but his hands pinning your arms prevented such an escape.

"then your just as fucked up as i am, aren't you? you dirty little human." He purred semi-affectionately, leaning down and biting your neck, making you cry out in real pain.

"Y-YES!" Your toes curled up in pain-infused pleasure, making your eyes water up. Sans paused, peering up at you concerned.

*shit. to much? sounded like i really hurt her..

You noticed him hesitate and pressed into him reassuringly. "D-don't hold back ok?" You managed after recovering. "B-because when it's my turn, I definitely wont." You tried glaring at him, attempting to sound somewhat threatening- but it ended up looking more like a pout, making sans smirk in amusement.

"your so cute." He quickly went back to nuzzling your neck, giving it a small lick, eager to test your bodies response to him. He wasn't disappointed as he successfully dragged a needy moan out of you, making his magic flare up as he listened to you beg for him. Unable to control himself he roughly cupped your jaw making you look up at him.

"What makes you think your even getting a turn?" He laughed, the lights in his eyes now gone, leaving his sockets looking dark and empty.

Almost threatening..

With a quick wave of his hand you suddenly found yourself naked as your clothes were instantly discarded. You couldn't even fathom how he had gotten you undressed so fast, finally figuring it must have something to do with his magic.

"mine." He growled low into your ear. His eye glowed a vibrant blue as he summoned his magic, illuminating the room and giving it an enchanted atmosphere. The surge of power sent shivers down your spine, making you excited and nervous all at the same time.

You flinched as you felt something press up against your entrance, and without any warning forcefully shoved itself inside of you. You let out a small cry as sans didn't even wait for you to adjust to him. His breath hitched as he slowly pulled out and quickly pushed himself back in, letting out a low growl of satisfaction as he was now fully sheathed inside of you.

"yesss~..." He breath out, doing his best to compose himself. He found it was harder to stay in control as he gazed down at you, watching the cute faces you made as he fucked you. The skeleton quickly began to pick up speed, finding a steady rhythm as he hungrily bucked his hips into you.

'S-SANS!~" You let out a loud moan, wrapping your legs around his waist as he continued to pound into your sensitive wet pussy. His thick monster dick filled you completely, hitting a spot deep within you that made you quiver in pleasure, completely submitting to him.

"your perfect like this." He breathed, hardly able to focus in his lust infused haze. His grip on your arms loosened, his hands finding their way to your hips and giving them a hard squeeze, feeling possessive as his fingers dug deep into your delicate skin. "my beautiful little human."

The affectionate tone of his voice made you shiver, your inner muscles tightening around him in response, successfully pulling a grunt from the skeleton. Now that your hands were free they immediately shot up to wrap around his neck, pulling him down closer to you. The position made his thrusts a little more clumsy, but you needed to feel him against you rite now. The contact of your body against him was comforting and made you feel safe.

"S-sans?" You called for him, making him pull back to look at you. Despite his lack of lips you didn't hesitate as you leaned in to place a meaningful kiss on his mouth, surprised to be greeted by a spark of magic in response.

Sans pulled back, breaking the kiss as he shifted to get in a more comfortable position. You let out a whine of protest and grabbed onto the fabric of his jacket, stopping him. "A-again." You blushed a deep red as you pulled him back down towards you, being rewarded with the pleasurable sensation of his magic on your sensitive lips.

His hand wondered to the back of your head, tangling his fingers up in your hair before bunching it up in his fist and yanking you backwards harshly.

"so greedy." He scolded seductively. His thrusts becoming harder and more eager, making a point to assert his dominance over you as he pounded into your pulsing core relentlessly.

"S-sssaa-aaanss!" You felt a searing heat rise deep within you, sans could sense it as well as you grid into him every time his hips thrusted into yours. Grabbing your thighs he shoved them apart even more, his movements becoming more primal as he watched you wither in pleasure underneath him.

"that's right. come for me, little human." He pulled back, his gaze wandering down to where you were connected, slowly reaching out he gently brushed his thumb over your clit, grinning as you cried out at his touch. Eagerly he did it again, his mouth hanging open slightly as he stared hungrily at your glistening sex.

Following his gaze you blushed a bright crimson, seeing the way he was completely fixated on your pussy. Suddenly making you feel extremely self-conscience you reflexively attempted to squeezed your legs together to block his view, instantly regretting it as your thighs were yanked apart and spread wide for him to see.

"let me watch." He growled possessively. "i wanna see you come." the deep rumble of his voice sent a shiver of excitement through your core, making you tighten all around him as you complied with his demand.

"SAANNSSS!~" Finally you were pushed over the edge, falling limp in the skeletons grip as your senses overflowed with paralyzing waves pleasure.

"f-fuck." Sans slowed his pace as he felt your muscles contracted around him, feeling himself come undone as his own orgasm rocked through him. You felt his cock twitch inside you as he emptied his magic seed into you, your walls stretching to accommodate the excessive amount of cum. "take it for me girl.' He bucked as far into you as he possibly could, holding you there until his magic was leaking out of your pussy.

"perfect." The pressure of sans cock inside you abruptly disappeared, his grip on your hips loosened, lazily sliding off of you to collapse onto the bed. Exhausted, he lazily slung his arm around you, pulling your naked body into him. You let out a satisfied hum, burring your face into his jacket. His sockets closed, running his phalanges through your hair, listening to your rapid heartbeat slow as you began to come down from your high.

"D-damn.." You breathed, finally finding your voice. "What a hell of a study session.." You propped yourself up to look at Sans suspiciously. "You got a masters degree in biology or something?" You managed to ask once recovering.

Sans peered open an eye, grinning at you teasingly. "nah. your just easy to figure out." his grip on your hair tightened, leaning your head up to look at him. "Maybe next time i'll teach you more about monster anatomy instead."

"Promise?" You glanced up at him expectantly.

"i don't make promises. but you can take my word for it when i say i'm not done with you yet." He leaned in, pressing his mouth against your lips in a silent promise. You felt your heart flutter in excitement, eagerly anticipating in seeing him again.

**Notes for the Chapter:**

> thanks for all the support on the last chapter! Here's your reward for being so patient with me, you dirty little sinners. ;D Not sure if im going to continue this series or just end it here. I mean who doesn't want kinky skeleton sex? But im also low on ideas. Got any kinks you'd wanna see? Leave a comment and a kudos and thanks for reading! <3


	3. Sans

Holyshit.

Your body was so sensitive right now.

Your shirt suddenly felt to tight as the fabric rubbed against your nipples, making you wiggle in your clothes as you craved the stimulation.

O-Oh god.

That felt so good.  
  
But it wasn't enough.

You needed something more.

Slowly your gaze wondered to Sans, who was sitting at the opposite end of the bed, observing you with a look on his face that sent a shiver down your spine and straight into your core.

Damn.

You weren't just receptive physically, but emotionally as well.

Sans hadn't even touched you yet and already this was the absolute horniest you've ever been for anyone in your whole life.

The way your skeleton was staring you down so hungrily, with such want in his eyes, was seriously hot as hell.

You couldn't look away, feeling like prey being hunted as he suddenly got up from his spot on the bed, and started crawling towards you with purpose.

Your heart skipped a beat.

Suddenly Sans was ontop of you, pinning you down below him with such force it scared you slightly.

Your fear was immediately replaced with want as your vertebae snaked an arm around your waist and up your back, pushing the fabric of your shirt up along with it.

Oh~

The way his bones felt against your skin was so foreign and weird, but in a good way.

It was different than being touched by human skin, his hands were smooth but hard, and much larger in comparison.

It felt more like getting a massage than being groped with the way he was grabbing you.

His phalanges wrapped around your tiny frame and traced along your spine and down to your butt, grabbing handfuls of flesh as he squeezed your cheeks, forcing your hips to roll up into his and grind into him.

Ngh~

Oh yea, that felt amazing~

The casual posessive way he was handling your body got you so wet.

Your were positive he knew exactly what he doing to you because not a second after you thought it he shoved your legs apart with his own, grinding you down onto his femur with fervor.

A-Ahh!

F-Fuck yes!

You arched up into him, pressing down onto him as he lead you into a slow grind.

"S-Sans~" Your voice was weak with want, your senses overwhelmed yet starved as the friction your monster was giving you just wasn't enough.

Sans knew it to, but prefered to tease you, liking the faces you made when he messed with you like this.

"relax sweetheart, i'll take care of ya'." He promised, removing his hand from it's place on your ass to lazily travel over the soft skin of your stomach, and up to your ribs.

You let out a sharp gasp as the skeleton tighted his grip around your body, digging his fingers into your skin as he ran his hands experiementally over your torso, feeling the bump of every rib as he explored the feeling of the bones in your body.

Sans did this often, intruiged and attracted to the differences and smillilarities between you.

Sometimes it wouldn't even be a sexual thing, he just genuinely liked to touch you, you were soft and squishy and made cute noises when he grabbed certain areas.

Honestly you loved it when he touched you like this.

It was so intimate but innocent and was more relaxing than anything when he explored you like this.

You closed your eyes and let out a hum of approval as Sans moved his hand from your stomach to your face, running a boney finger over your eyebrows, and against your eyelashes, seeming to like the sensation of it because he did it again before removing his hand to trace down along the curve of your neck.

Pressing his palm flat against your neck he took his other hand and pressed it to your chest, feeling both your heartbeat and pulse simultaneously as he listened to the calm flow of your breathing.

To an onlooker it might look weird, but it was atually pretty soothing, it felt like you were at the doctors getting a routine checkup, except better because your doctor was Sans, and who WOULDN'T want to get examined by him?

"open your mouth." Absently you obeyed, half expecting him to stick a popsical stick down your throat, but was pleasantly surprised to be met with the familliar feeling of his teeth against your lips, his ecto-tongue finding refuge as it snaked its way into your mouth and wrestled against your tongue for dominance.

Oh yea~

You loved kissing Sans.

Maybe it was the way his magic danced across your taste buds and lit up your nerves, but you always felt super comfortable with him.

There was a mutual curiosity between the two of you, neither feeling shy or reserved when being intimate like this.

Sans was always so laid back and cool, even in bed he'd casually lead the way and let you follow at your own pace.

Unlike previous relationships, there was never any pressure with Sans, he'd always know when to back off or slow down and never pushed you for more than what you were willing to give him.

Which was never a problem because you were willing to surrender your entire soul to the monster at the drop of a hat.

You were so focused on the way Sans translucent muscle felt against yours you almost didn't even notice the way he slipped his hand down the front of your pants and cupped your sex, dipping a finger in to see how ready you were for him.

"Ahh~"

Oh my god, were you ready.

"heh."

You let out a whine of protest as the skeleton pulled away, bringing a soaking finger to his mouth and running his tongue along the appendage to taste your juices.

L-Lewd.

"take off your pants." Your skeleton instructed, letting you up so you could wriggle your way out of the constricting fabric and and toss it onto the floor next to his sock collection.

"shirt to." He thoughtfully reminded you, and damn you couldn't get it off fast enough as you pulled it up and over your head and threw it aside to join your pants on the floor.

"eager?" He grinned, pinning your body against his as he retook his position ontop of you.

"Y-Yea." The slight pressure of his bones against you sent a shiver of nerves through you, making your stomach do flips as you watched him lower his waistband just enough to free the glowing blue erection hiding in his pants.

D-Damn.

This wasn't your first time with him, but everytime you saw it you couldn't help but blush and smile, grinning up at him like an idiot as you waited for him to make the first move.

Fuck.

It was rare for Sans to lose his cool, but everytime you looked at him like that he felt his control quickly slipping away from him.

He wasn't sure if it was a dominance thing or if he was just a pervert, but the way your eyes lit up when you were with him, the raw eagerness and excitment he saw behind them, completely puzzled, yet aroused him.

It reminded him just how weak you actually were to him.

You were fragile and soft, and something deep inside him wanted to break you.

To hurt you.

But he couldn't _._

He'd never let himself lose control like that, because down in his soul he also knew that he loved you and needed to protect you.

You were his human, and he cherished you.

"ready?"

Grabbing you tightly around the hips he ground you down onto him, his eye flaring a bright cyan as he felt your sopping cunt around his magic, making him hiss out through clenched teeth and bury his face into your neck.

Ok, wrong.

He was so, so, wrong.

He was weak.

Stars, he was so weak.

You were warm, and wet, and hugged him so tight.

The truth was you were the one that made him feel safe.

He knew that no matter what he'd never be alone, because he'd always have this.

He had you.

**Author's Note:**

> Second thing ive ever posted on here. Love the community on here so i am really hoping for comments and criticism. Please leave a comment and tell me what you would like to see or give some constructive criticism on what i can improve. Im a very busy person inrl and work alot so updates will be sporadic But please bear with me and thanks for reading! <3


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